The present invention relates to a method for correcting black defects (Opaque defects) in a semiconductor photomask using a Focussing Ion Beam (FIB) and an apparatus thereof.
Conventionally, irradiating a prescribed region of a photomask with a focussed ion beam and eliminating black defects using sputtering effect or gas-supported ion beam etching has been provided as a method for eliminating remaining shading film material (black defects) for other than the prescribed pattern of a semiconductor photomask. First, a description is given based on FIG. 4 of an outline of a focussed ion beam processing apparatus of technology preceding the present invention. FIG. 1 shows an ion source 1, an ion beam 2, an electrostatic optical system 3, a deflecting system 4, a sample stage 5 for mounting a sample 10, a second charged particle feeding optical system 6 for collecting secondary charged particles 11, a mass spectrometer or secondary electron detector for detecting the presence and mass of the secondary charged particles 11, a signal processor 8, and a display 9. In the present invention, the sample 10 is a photomask having black defects 21, 22 and 23 where typically shading film material of chrome or chrome oxide exists on the photomask as shown in FIG. 1 during production. The present invention relates to technology for irradiating the defective portions with an ion beam and carrying out an eliminating process. Preferably, just the black defect region should be irradiated with a beam. There are problems, however, where if regions other than the black defect portions such as a foundation substrate of glass etc. (ex.Quartz etc.) are irradiated with a beam then these portions will be damaged or the transmission factor of glass portions may be lowered due to the injection of the ion beam. Defect correction is extremely important from the point of view of productivity because as the photomask is used to produce a large number of semiconductors, any defect in the photomask itself will be reproduced in any semiconductors made using such a mask. In Japanese Patent Publication No. Sho. 62-60699, a scanned image of the processing surface is obtained and a scanning region is specified based on this scanned image so that regions other than the black defects are not irradiated with the ion beam. Further, the black defect regions change with the process from one minute to the next. The image is therefore captured in stages so as to execute the process while confirming the black defect regions and therefore provide fine grain processing. However, setting scanning region based images for a number of stages cannot be said to be sufficient. Therefore, recently, in a process where processing is carried out by irradiating with a beam for a fixed time to give a fine grain, secondary charged particles indicating the presence of the workpiece every processing dot are monitored, it is determined that black defects are not already present when a value in excess of a reference value is not detected and the process is not carried out next time. However, this determination is based just on detection information for individual dots and new problems with processing defects occur such as over-etching and residual etching as a result of this determination being easily influenced by random noise.
It is therefore the object of the present invention to provide a photomask black defect correction method for monitoring secondary charged particles to determine the presence of black defects in portions irradiated with an ion beam where fine grain processing can be implemented for each dot so that highly reliable end point detection information which is not easily influenced by noise is obtained so that defective processing such as over-etching and residual etching does not occur.
In the present invention, a dot matrix taking a spot of an ion beam at a processing surface as one dot is set at the processing surface, the presence or absence of secondary charged particles, indicating the presence of a black defect, discharged from a plurality of dots including a noted dot and associated peripheral dots is detected while eliminating black defects by irradiating each dot of a processing region with an ion beam for a fixed time, calculating a physical quantity corresponding to this presence based on this detection information is carried out, and completion of processing of this noted dot is determined when this value is less than or equal to a reference value.
The present invention provides improved reliability in end point determination by taking a plurality of dots as a dot group in a matrix centered about a noted dot and then making determinations by making comparisons with a reference value while assigning different weightings to each dot according to the distance of each dot from the noted dot.
At defect regions coming into contact with non-processed regions such as a normal pattern region, etc., highly reliable end point determinations can also be achieved for special regions by carrying out determinations on noted dots to the exclusion of dots for detecting the presence of secondary charged particles indicating the presence of a workpiece.